This invention relates to a north finding system implemented as gyrocompassing apparatus intended for use on ground based equipment such as antenna pedestals, projectile launchers, and like equipment requiring north reference information. The gyrocompassing implementation enables utilization of high grade state-of-the-art inertial sensors and yet achieves the desired results economically. The north finding system provides high accuracy with fast reaction time over a wide temperature range without the need for auxiliary equipment such as heaters or the like. The system is specifically configured to tolerate settling and/or oscillatory base motion without additional reaction time or degradation of gyrocompassing accuracy, as is the case with prior art north finding systems. This feature is important in applications where host equipment base settling, personnel movement, and wind factors for example, impose a dynamic gyrocompassing environment.
One relatively low cost configuration considered for a north finding system for the purposes required features a strapdown package using a conventional single spinning wheel gyroscope (gyro). While such an arrangement is adequate with small tilt angles and with no base motion, or with a relatively non-stringent gyrocompassing. accuracy, it suffers from temperature sensitive mass unbalance effects which cause apparent gyro bias shifts for each specific combination of temperature and tilt. Without temperature control, which adds to the complexity of the apparatus, only a few degrees of deviation from the reference causes gyrocompassing errors which are unacceptable for the purposes intended. Further, this temperature sensitivity of rotating mass gyros affects both accuracy and reaction time. Accordingly, an important feature of the present invention is that it is virtually temperature insensitive and exhibits no mass unbalance effects.
Ground based installations for which the present invention is intended have a tendency to settle when emplaced. Settling can be caused either by a slow sinking of a vehicle or other structure into the terrain, or by the suspension system supporting the installation compressing from its running state. Any sensed angular rate contaminates the earth rate reading which a strapdown gyro measures to find north. A relatively small settling rate can cause a substantial gyrocompassing error which is magnified at higher latitudes. Accordingly, high performance accelerometers coupled with a sophisticated software program have been used to compensate for the settling angle.
Rotational vibration creates another problem observed in some north seeking arrangements. This problem can be obviated by using a ring laser gyro with a high bandwidth and low quantization, which allows accurate motion reconstruction during high angular rate movements to permit the ring laser gyro read-out electronics to operate at relatively high angular rates.
It should also be noted that lateral vibrational inputs measured by accelerometers used to compensate gyro outputs do not present a problem. These varying measurements average to an acceptable level over a period of time. Another advantage of arrangements of the type described is the virtually instant turn on time, independent of temperature. Thus, these arrangements have been observed to be ready only three seconds after applying power, thereby allowing a longer gyrocompassing average time then conventional arrangements which take much longer to reach operational steady state spin rate and specified accuracy.
A properly designed three gimbal platform arrangement, constructed around spinning wheel inertial rate gyros which are levelled, may serve the purposes intended. In an arrangement of this type the levelled platform isolates the north seeking gyro from tilt and rotational movements. The arrangement disclosed in commonly assigned U.S. Pat. No. 4,686,771 issued on Aug. 18, 1987 to Thomas Beneventano, et al, also serves the purposes intended. However, other arrangements have been found to be more desirable than that taught by the prior art as aforenoted. One such arrangement is disclosed in commonly assigned co-pending U.S. application Ser. No. 327,874 filed on Mar. 23, 1987 by Thomas Beneventan now U.S. Pat. No. 4,945,647. This arrangement requires independent means for measuring the relative gimbal angle and a resolver system is used for that purpose. The present invention is an improvement over that disclosed in the aforenoted application in that the need for independently measuring the relative gimbal angle is eliminated. This is accomplished by skewing the input axis of the gyro. In this regard, note that during gimbal rotation a component of said rotation is applied to the gyro input axis. Absolute position alignment between the gyro gimbal and case is required, but this is an easier task than measuring the relative gimbal angle as has heretofore been necessary.